Although more than 95% of active climate scientists attribute recent global warming to human causes (1, 2) and most of the general public accepts that climate change is occurring, only about half of U.S. adults believe that human activity is the predominant cause (3), which is the lowest among 20 nations polled in 2014 (4). We examine how this societal debate affects science classrooms and find that, whereas most U.S. science teachers include climate science in their courses, their insufficient grasp of the science may hinder effective teaching. Mirroring some actors in the societal debate over climate change, many teachers repeat scientifically unsupported claims in class. Greater attention to teachers' knowledge, but also values, is critical.